1. Field of the Invention
The invention relates to a ceramic substrate support chuck for supporting a semiconductor wafer within a semiconductor processing system. More particularly, the invention relates to an improved multi-layered and doped ceramic electrostatic chuck, and method of fabricating same, for decreasing resistivity of a portion of the chuck for operation at room temperature by the Johnsen-Rahbek effect.
2. Description of the Background Art
Substrate support chucks are widely used to support substrates within semiconductor processing systems. A particular type of chuck used in high temperature semiconductor processing systems such as high temperature, physical vapor deposition (PVD) is a ceramic electrostatic chuck. These chucks are used to retain semiconductor wafers, or other substrates, in a stationary position in the process chamber during processing. FIG. 1 depicts a typical ceramic electrostatic chuck 100. Such electrostatic chucks contain one or more electrodes 104 and 106, respectively embedded within a unitary ceramic chuck body 102. The ceramic chuck body 102 is, for example, fabricated of aluminum nitride or boron nitride or alumina doped with a metal oxide such as titanium oxide or chromium oxide or some other ceramic material with similar resistive properties. This form of ceramic is partially conductive at high temperatures.
In use, a wafer rests flush against the surface of the chuck body as a chucking voltage is applied to the electrodes. Because of the conductive nature of the ceramic material at high temperatures, the wafer is primarily retained against the ceramic support by the Johnsen-Rahbek effect. The Johnsen-Rahbek effect establishes a small, but highly effective current flow between the substrate support surface and the substrate being retained. As such, a chucking force that is much greater than the force generated by a purely Coulombic effect electrostatic chuck retains the substrate to the support surface. Johnsen-Rahbek chucks are disclosed in U.S. Pat. Nos. 5,117,121, issued May 26, 1992 and 5,463,526 issued Oct. 31, 1995.
One disadvantage of using a chuck body fabricated from ceramic is that the resistivity of the chuck changes as a function of temperature. During wafer processing, the chuck is subjected to a wide range of temperatures, e.g., in the range of 20.degree.-150.degree. C. and could be as high as 300.degree.-400.degree. C. for other types of processes. At room temperature (approximately 20.degree. C.) the resistivity of the chuck is on the order of approximately 10.sup.14 ohm-cm and decrease to approximately 10.sup.13 ohm-cm at temperatures around 150.degree. C. This decrease in resistivity promotes a satisfactory chucking force via the Johnsen-Rahbek effect. However, as the temperature rises in the chamber, the resistivity level continues to decrease. As such, the current flow in the chuck body, hence the wafer, attributed to the Johnsen-Rahbek effect becomes larger. A wafer or similar substrate clamped to the support surface of a chuck having a high current flow can be damaged to the point of having a lower yield or being totally unusable.
Controlling the resistivity of the bulk material is essential to fabricating and using a Johnsen-Rahbek effect electrostatic chuck. The two parameters that are available for controlling resistivity are process temperature and the materials of the chuck. Unfortunately, decreasing process temperature does not provide an optimal resistivity to establish the Johnsen-Rahbek effect. Therefore, altering the materials comprising the chuck body is necessary. Doping the bulk material is a viable solution; however, it is critical to control the amount of doping. For example, an electrostatic chuck having a highly doped bulk material generates excessive charges which can short chucking electrodes and impede dechucking due to residual charge buildup. Such a condition is not suitable for a Johnsen-Rahbek effect chuck.
Therefore, a need exists in the art for an apparatus that has a resistivity level suitable for establishing the Johnsen-Rahbek effect to electrostatically clamp a substrate to a support surface and a concomitant method of fabricating same.